This invention pertains generally to the field of optics and optical communication systems and particularly to optical modulators for fiber optic communication systems.
The utilization of optical fiber in communication networks is growing rapidly. Such networks require highly reliable and preferably low-cost optical switching and modulation devices. To achieve higher reliability and lower cost, substantial efforts have been made to produce optical switching devices using microelectromechanical system (MEMS) techniques. See, e.g., W. H. Juan, et al., xe2x80x9cHigh aspect ratio Si vertical micromirror array for optical switch,xe2x80x9d J. Microelectromech Syst., Vol. (17), Nov. 2, 1998, pp. 207-213; Makoto Mita, et al., xe2x80x9cOptical and surface characterization of poly-Si replica mirrors for an optical fiber switch,xe2x80x9d Transducers ""99, pp. 332-335; S. S. Lee, et al., xe2x80x9cSurface-micromachined free-space fiber optic switches with integrated microactuators for optical fiber communication system,xe2x80x9d Transducers ""97, pp. 85-88; A. Miller, et al., xe2x80x9cAn electromagnetic MEMS 2xc3x972 fiber optic bypass switch,xe2x80x9d Transducers ""97, pp. 89-92; C. Marxer, et al., xe2x80x9cVertical mirrors fabricated by reactive ion etching for fiber optical switch applications,xe2x80x9d IEEE Int. Conf. On MEMS ""97, pp. 49-54.
The most common switching elements used in present MEMS optical switching networks are micromirrors. A significant challenge in the production of MEMS micromirrors is the achievement of sufficiently high reflectivity and smoothness of the mirror surfaces. The best reflectivity that apparently has been reported to date for MEMS micromirrors has been about 85% (xe2x88x920.71 dB), which is achieved by coating gold on a silicon mirror. The roughness of the mirror is about 5 nm with proper fabrication processes. See, W. H. Juan, et al., supra. The use of mirrors in micro-optical systems presents particular problems for planar systems in which the light travels parallel to the plane of the substrate. Planar systems are desirable because they offer the highest potential level of integration by allowing an entire optical bench to be implemented on a single semiconductor chip. However, the use of mirrors as the optical switching elements requires that these optical elements must either be bulk micromachined into the silicon substrate, or be surface micromachined from deposited thin films which then must be flipped up.
In accordance with the present invention, a micromechanical optical modulator can be constructed by standard MEMS batch fabrication techniques on conventional planar substrates, enabling relatively low cost production and high reliability. The optical modulators may be utilized in optical communication systems for purposes such as on/off switches, routing switches, switched modulators and in various types of sensors, such as accelerometers. Optical switching devices embodying the invention may be incorporated in a communication system with low insertion loss and rapid switching times that are comparable to or better than conventional switching elements now used in fiber optic communication systems.
The micromechanical optical modulator of the invention may incorporate an input optical waveguide with an exit face from which a light beam can exit the waveguide, and an output optical waveguide with an entrance face spaced from the exit face of the input optical waveguide to receive a light beam exiting from the exit face on a beam path. Such optical waveguides can include, but are not limited to, optical fibers of the type utilized in fiber optic communication systems. A phase shifting gate is mounted between the input optical waveguide and the output optical waveguide. The phase shifting gate includes a light transmissive panel and can be moved between at least two positions. The light transmissive panel has at least one section having outer surfaces. The light transmissive panel may have two (or more) sections with one thicker section having a thickness greater than that of another thinner section. The phase shifting gate is translatable between at least two positions. For a panel having two sections, in one position the thinner section is interposed in the beam path between the input and output optical waveguides, whereas in the other position of the gate, the thicker section is interposed in the beam path. For a panel having a single section, the panel is interposed in the beam path in one position and out of the beam path in another position. The spacing between the exit face of the input optical waveguide and the adjacent surfaces of the sections of the panel, the spacing between the entrance face of the output optical waveguide and the adjacent surfaces of the sections of the panel, and the index of refraction of the light transmissive panel are selected such that, for a selected wavelength of light in the beam, in one of the positions of the gate the beam is transmitted through or past the section of the panel and in a second position of the gate the light in the beam is substantially reflected by interference effects.
An actuator may be connected to the phase shifting gate to drive it between its two positions. A micromechanical spring may be mounted to a substrate and connected to the gate to support the gate for lateral motion and to resiliently bias the gate back to an initial position. The gate may thus be switched from a position in which it transmits the light beam from the input optical waveguide to the output optical waveguide to the second position to block the beam, thereby effectively acting as an on/off switch. The switch can be actuated rapidly to provide pulsed on and off switching of the beam. In addition, as the gate transition regionxe2x80x94at which the thicker section and the thinner section of the panel meetxe2x80x94enters and progressively moves across the width of the beam between the input and output optical waveguides, the intensity of the transmitted beam will be progressively reduced or increased, depending on the direction of travel of the gate. Thus, very precise sensing of the position of the gate can be obtained by detecting the intensity of the transmitted beam. The optical modulator of the invention may thus be incorporated in microsensors such as pressure sensors (with the gate connected to a diaphragm that deflects with pressure changes), strain sensors, and accelerometers. An accelerometer may be formed by connecting a proof mass to a phase shifting gate supported by a spring so that the gate is displaced by the force exerted by the proof mass during accelerations.
The optical modulator of the invention may also be incorporated in communication systems to redirect light beams, utilizing input and output optical waveguides that are oriented at a non-perpendicular angle to the surfaces of the light transmissive panel of the phase shifting gate. In one of the positions of the gate the panel is out of the beam path or a first section of the panel is in a beam path from a first input optical waveguide to the entrance face of a first output optical waveguide on the other side of the gate, providing a continuous beam transmission path from the first input optical waveguide to the first output optical waveguide. In a second position of the gate the panel or a panel section of different thickness than the first section is interposed in the beam between the first input and output optical waveguides such that the light in the beam is reflected by interference effects and is directed to the entrance face of a second output optical waveguide, completing a redirected transmission path from the first input optical waveguide to the second output optical waveguide, and blocking transmission to the first output optical waveguide. A second input optical waveguide may be mounted to transmit light through a first section of the panel to the second output optical waveguide in the first position of the gate, and to have the light in the beam reflected by the panel in the second position of the gate to the first output optical waveguide, thus completing a new light transmission path between the second input optical waveguide to the first output optical waveguide.
Various materials are suitable for the light transmissive gate. Crystalline silicon, for example, is well suited for transmission of light in the infrared wavelengths commonly used for optical communication systems. A wavelength of light commonly used in optical communications is 1.55 xcexcm, at which silicon is essentially transparent. Such micromechanical silicon gates can be readily constructed by well developed MEMS processing techniques for silicon microstructures. No additional surface finishing or coating steps (such as gold coating) are required, as is typically the case with micromirrors.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.